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Laser Induced Surface Topography

Lasers are being adopted for precision micro-scale machining and surface conditioning across a variety of industries such as semiconductors, LED and bio-medical device manufacturing. Laser ablation creates surfaces that are a challenge for conventional profilers, because of reasons such as rapidly changing geometry, high aspect ratios and very high roughness, to name a few. In this applications note we have taken some of these challenges and shown how the Zeta3D™ Optical Profilers can be used to quantify surfaces in True Color, irrespective of their roughness, material or dimensions.

Industry Challenges

Non-destructive testing

Variety of materials

High Aspect Ratio measurement

Accurate data

Quantifiable & actionable data

Convenience

Speed

Cost Savings

The Zeta3D™ Advantage

Non-contact optical profiler

Transparent, opaque, and coated samples

High efficiency illuminator and special algorithm

Broad band and true color (no coherent artifacts)

Automated height, CD and volume reporting

No sample preparation

Fast scan time and user friendly GUI Fully automated

critical dimension (cd) & volume quantification with true color

Technology

In 2009, Zeta developed the 1st Confocal Grid Structured Illumination (CGSI) based optical profiler. This technique evolved confocal microscopy to enable measurement of di cult surfaces and also image in true color. Zeta Instruments’ patented ZDot™ imaging technology (commercial name for CGSI) is immune to the shortcomings of traditional microscopy techniques, and is the perfect solution to laser cutting applications.

A leading university is researching pulsing wattage laser power and how it correlates to the depth and shape of the crater on a variety of materials. Shown on the left are craters created on a Tungsten surface coated with Aluminum, scanned with the ZDot™ mode.

Flexible circuits are widely used in automotive, medical, industrial, wearable, and consumer products. These circuits are built on large sheets of copper foil and other laminated flexible substrates which are easily deformed and difficult to handle. This poses challenges to traditional optical metrology tools such as white light interferometer. The Zeta 3D optical profiler is able to automatically measure critical dimensions such as the diameter and depth of Laser Drilled Blind Vias on these flexible substrates at high throughput in a production environment. This via is 100 µm in diameter and 28 µm in depth. The ZDot™ (CGSI) mode was used to create the true color image shown above. Note the color variation in the surface around the via.

The semiconductor industry is adopting soft laser Wafer ID marking techniques,due to the waferwarpage caused bythe traditional hard laser marks. For fabs operating at the < 16nm node, wafer real estate is precious. Ideally wafer ID’s should be shallow, while at the same time having enough contrast to be read by optical character recognition devices.

The challenge of measuring such features is to automatically locate these shallow markings on a bare wafer and profile them with nm resolution at fast throughput. The Zeta3D™ optical profiler is able to measure the nanometer ripples caused by the laser, as highlighted by the red marks. The ZX100 option was used to create the image shown above.

In thin film solar cells, electrical isolation is important and for this, trenches are created to isolate the P and N-doped regions of the solar cell. In some thin film cells, the laser scribe shout not breach the underlying glass substrate and must be contained within the thin films. The trench may also be heavily textured to increase the light absorption. Controlling the trench dimensions and the surface texture is key to improving the efficiency of the solar cell.

For this, ultrafast pulsing lasers are used to produce cleaner, sharper micro-channels. The challenge is to automatically locate these trenches on large panels and provide critical information about their roughness, depth and width. The scribe shown above is 74 µm in width and 0.50 µm in depth. The Zeta3D™ profiler uses a high precision motorized XY stage and pattern recognition software to automatically detect the scribe lines and scan multiple locations on the solar cell.

Microfluidic devices o er great potential for performing biological and chemical assays under carefully controlled conditions by integrating multiple fluidic functions onto a single device. Lithography techniques are being replaced by laser ablation which consists in focusing very short laser pulses to a small area of the substrate. The lateral size of the crater is controlled by the laser focus size and the depth by adjusting the number of pulses. Heat induced deformations, irregular shapes, large side wall slope angles, low image contrast and low reflectivity are all the result of the laser processing and these pose measurement challenges for the traditional interferometer and confocal profilers. The Zeta3D Feature Detection software and ZDot™ provides advantages over. The channel shown on the left is 534μm wide and 438μm in deep.

Laser ablated trenches in Indium-tin oxide (ITO), typically used in flat panel displays and organics based electronics such as transparent electrodes and organic light emitting devices (OLED), o er possibility for high e ciency structuring of transparent conductors on glass and other substrates. Laser direct write (LDW) is a maskless, dry process, that allows easy change of the contact pattern. The well defined edges and good electrical isolation at a short separation between conductor lines are required. Sharp edges are especially important when the distance between the conductor lines shrinks down to 10 μm. A laser confocal tool will show interference artifacts on such transparent samples with irregular topography, whereas the Zeta3D™ Optical Profiler using the ZDot™ mode is able to acquire meaningful data on such samples. This scribe is 8.6μm in width and 0.2μm in depth.

Laser scribing is a widely used technique to separate a LED wafer into individual dies. The scribe process creates a deep and narrowing trench and leaves behind a lot of debris. Such a geometry precludes the use of normal imaging tools such as interferometers and laser confocal microscopes. Zeta’s high e ciency optical design and the ZDot™ mode allows the maximum possible light to be collected from such a sample. The scribe shown above is 9.1 μm in diameter and 15.84 μm in depth, with very rough, tapering side walls.

Defect review and root cause analysis is an integral part of any failure analysis lab. However, finding the defect on the microscope and transferring the samples to AFM or SEM becomes a challenge if the defect cannot be accurately located for further analysis. Laser scribes generate minimal debris and can accurately mark defects on a variety of substrates. Shown above is a test laser scribe generated on thin film coated Silicon surface. The True Color Zeta3D™ information is used to calibrate and adjust the laser for proper scribing. High sample Roughness, debris, and low reflectivity present challenges for conventional interferometers. The ZDot™ mode on the Zeta Optical Profilers can easily tackle such surfaces.